1. Field of the Invention
The present invention relates to a method for producing powdered wetproofed material for gas-diffusion electrodes, from electroconductive powder especially carbon black and from wetproofing agents.
2. Description of the Prior Art
The use of carbon and other catalytically active materials and wetproofing substances (polytetrafluoroethylene, poly ethylene, polypropylene, elastomers and others) for the manufacture of gas-diffusion electrodes is well known. A characteristic feature of the known methods for producing gas-diffusion electrodes containing such materials is that the catalytically active materials (platinum black, silver, activated carbon and others) are also wetproofed and the wetproofing process proceeds during or after the preparation of the electrodes (U.S. Pat. Nos. 3,348,974, 3,386,685, 3,405,010, 3,423,247; French Pat. No. 1,529,016; East German Pat. No. 56,837).
These methods possess a number of disadvantages. A considerable part of the active surface area of the catalyst is blocked during wetproofing of the electrodes and is excluded from the electrochemical reaction. This leads to a decrease of the electrode effficiency. In addition, in each particular case suitable wetproofing agents must be found and optimum conditions under which the wetproofing process is to be performed must be determined. The gas supply in such electrodes strongly depends on the porous structure of the catalytically active material and it is not always possible to find the appropriate conditions ensuring a satisfactory gas supply in the presence of electrochemically inert gases such as nitrogen (from the air).
This is also the case with respect to Japanese Pat. Publication 7,012,562 (Matsushita) which discloses mixing dry powders of carbon (catalyst powder) and resin powder. The description of the Matsushita publication on page 1, lines 28-34 states that the known use of a polytetrafluoroethylene (PTFE) dispersion for wetproofing results in a very short lifetime for the electrode. Matsushita attempts to improve this by mixing dry carbon and PTFE powders and using a further polymeric binder. In the process according to this Japanese Patent, catalyst powder (carbon powder) is mechanically mixed with fluoro resin powder; the mixture (as a powder or as a pressure-molded tablet) is heat-treated (at 350.degree. C) and then pulverized into powder. The pulverized powder is mixed with a solution of synthetic resin (polyvinyl chloride in tetrahydrofuran) as a binder, press-molded and then dried. This Japanese process differs both in the procedure used and the material treated, compared to the method of the present invention. In the method of the present invention, a suspension of electrically-conductive powder (e.g. carbon black) in a liquid is mixed with a dispersion of wetproofing agent and under these conditions uniform homigenization and combination of the particles takes place, the procedure itself thus differing from the method of the Japanese Patent. Further, in the present method, the conductive powder is itself not active as a catalyst, whereas in the Japanese method the catalytically active carbon powder is wetproofed. Thus, in the Japanese patent, a substantial part of the active surface of the catalyst is blocked, which leads to a decrease of the electrochemical activity. The ultimately formed electrode of the Japanese patent is prepared from this wetproofed catalytically active material, while according to the present method, an electrochemically inactive material (e.g. carbon black) is wetproofed and the obtained wetproofed material may be used in combination with a proper catalyst (not subjected to wetproofing) for the ultimate preparation of gas-diffusion electrodes.
It also may be seen from the description of said Japanese publication 7,012,562 that the wetproofed material is additionally mixed with a solution of synthetic resin (polyvinyl chloride in tetrahydrofuran) as a binder, which can lead to a deterioration of hydrophobic properties and to an additional decrease of the electrochemical activity of the catalyst, while the wetproofed material obtained according to the present method does not need an additional binder, because it is used as a binder itself.
U.S. Pat. No. 3,432,355 (Niedrach, et al) is typical of other prior art, and discloses an electrode consisting of platinum black and PTFE pressed on a nickel screen. The electrode is made by applying an aqueous slurry of platinum black and PTFE to a PTFE film. The water is then evaporated. The resulting wetproofed platinum black is not sufficiently hyddrophobic, thus requiring use of the PTFE film.
According to the description and claims of the said Niedrach, et al. patent, the electrodes are prepared in the following way. On a PTFE surface film an aqueous slurry of platinum black and PTFE dispersion is spread, the water is evaporated and the temperature is increased to dispel the emulsifying agent. The electrode consists of two such layers (one of them without PTFE film) sintered on the screen.
Platinum black wetproofed in this way does not have enough hydrophobic properties to stop the leakage of the electrolyte through the electrode. The lack of sufficient hydrophobic properties compelled Niedrach, et al. to cover the gas side of electrodes with a thin gas-permeable film consisting essentially of PTFE. This film renders the electrode impervious to the aqueous electrolyte (U.S. Pat. No. 3,432,355, page 1, lines 59-61). The use of such PTFE film is absolutely necessary because platinum black wetproofed according to the method described by Niedrach, et al. is not hydrophobic enough; when electrodes without such a film are investigated two very harmful effects are observed: flooding of electrodes and penetration of the electrolyte through the electrodes. This is clearly explained in the article of the same authors, (Niedrach, L. W. and Alford, H. R., J. Electrochem. Soc. 1965, Vol. 112, No. 2, p. 117-124), in which a detailed investigation of electrodes described in U.S. Pat. No. 3,432,355 is given. Even the presence of PTFE film cannot completely stop leakage and said article states that "a small amount of seepage has, however, been observed with all the electrodes tested to date" (page 119, right column, lines 36-38). Therefore, platinum black wetproofed with PTFE according to the method of Niedrach, et al. is not hydrophobic enough to stop leakage of electrolyte through the electode and leakage is not completely stopped even with the addition of a thin PTFE film. Thus, it follows that according to the method of Niedrach, et al. a material with sufficiently hydrophobic properties cannot be obtained.
The method described in U.S. Pat. No. 3,272,897 (Herman, et al) involves use of a polymer obtained upon contact of two immiscible solutions. This method is not suitable for forming polytetrafluoroethylene, which is the preferred wetproofing material for gas-diffusion electrodes.
According to the description of U.S. Pat. No. 3,335,034 (Laurent), the catalyst (active carbon, silver or nickel) is wetproofed with a solution of polystyrene. One side of the pellet formed from this material is additionally coated with silver catalyst. This patent does not disclose data for the lifetime of the electrodes, and the reference electrode against which the polarization is measured is not specified.
An electrode described in U.S. Pat. No. 3,459,652 (Grangaard) consists of catalyst (active carbon) wetproofed with paraffin (intimately mixed with and adsorbed by the carbon particles). This patent does not disclose data for the polarization and current densities of the resulting electrodes.
U.S. Pat. No. 3,537,906 (Sindorf) concerns an oxygen electrode consisting of a catalyst (silver powder) wetproofed with a PTFE dispersion. There are not data given for the lifetime of the resulting electrodes or for the polarization when the working gas is oxygen from the air.
U.S. Pat. No. 3,549,423 (Grubb, et al) concerns oxygen electrodes consisting of a catalyst (platinum) wetproofed with PTFE and a hydrophobic polymer (PTFE) film. There are no data disclosed for the lifetime of the electrodes.
According to the description and claims of U.S. Pat. No. 3,351,494 (Batzold), the disclosed method of preparing electrodes consists of mixing a catalyst of the platinum group metals with a wetproofed base. The wetproofed base is prepared by soaking the base material (asbestos, carbon, mica or alumina) in a wetproofing solution (halogenated silanes). The particulate base material used has diameters within the range of from about 50 - 350 microns. Apparently the base material is soaked, because it consists of porous particles. In the method of the present invention, the particles, e.g. carbon black, have dimensions less than 0.1 micron and are without pores.
In the above prior art, it is the catalytically active material which is wetproofed, and this results in blocking a considerable part of the active surface of the catalyst, leading to a decrease in the electrochemical activity of the electrode prepared therefrom. In contrast, the present invention provides a material in which an electrochemically inactive material (e.g. carbon black), which is not catalytically active, is wetproofed. The resulting material has uniform hydrophobic properties and may be combined with a catalytically active material (not itself subjected to wetproofing) to produce an electrode having an extremely long lifetime.